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Every year, millions of women are affected by the pain and discomfort of urinary tract infections or UTIs. For some, the infections may return even after treatment, recurring two or more times within a six-month period.

Now, researchers have found that recurrent UTIs may be caused by particular strains of Escherichia coli bacteria that can travel back and forth between the gut and the urinary tract with relative ease. The research appears in the 8 May issue of the journal Science Translational Medicine.

The risk of recurrent UTIs in women increases with each infection, according to studies conducted in 2005 and 2007. The exact cause of recurring infections is still unclear. Some research has focused on the bacteria’s resistance to antibiotic treatment, while other studies have examined whether UTIs weaken the cells lining the urinary tract, making them more vulnerable to repeated infection.

Bacteria growing in one habitat within the body will tend to get better at surviving in that particular environment. Thus, when E. coli invades and establishes itself in the bladder or urinary tract during infection, it’s assumed the bacteria gradually lose their ability grow well in other places such as the gut.

However, while studying E. coli strains isolated from a group of women with successive episodes of UTI, Swaine Chen at Washington University in St. Louis and colleagues found that strains can survive and persist in both the gut and urinary tract.

Using a variety of tests, including animal infection models and genetic sequencing, they found that in almost every case E. coli isolated from the patients’ guts and urine were identical.

Moreover, in some of the patients, E. coli strains changed completely between two UTI episodes. The “new” strain in the urinary tract took over the gut E. coli population as well, suggesting that the bacteria were well adapted to spending time in both places. If E. coli are able to move freely between the gut and urinary tract, the researchers say further studies are needed to determine the specific migration patterns of these bacteria between different sites within an individual and between individuals.

The results may influence current standards of care for UTIs. For example, sampling microbial communities from different body habitats may help researchers identify bacteria population shifts that predict new infections in patients with histories of UTI.

Links
Read the abstract, “Genomic Diversity and Fitness of E. coli Strains Recovered from the Intestinal and Urinary Tracts of Women With Recurrent Urinary Tract Infection,” by S.L. Chen and colleagues.